1. Field of the Present Invention
The present invention relates to an absolute position measuring device. More particularly, the present invention relates to an absolute position measuring device for measuring the position of a spindle in absolute format in a micrometer head, a micrometer, or a hole test, for example.
2. Description of the Related Art
In the small measuring instruments for measuring the length, size or angle, for example, the micrometer and the micrometer head, a measurement object is measured by detecting the information as to the relative movement amount of a movable member to a fixed member.
The methods for measuring the relative movement amount of the movable member to the fixed member include the increment type, which the present applicant has disclosed in Japanese patent examined publication No. Hei. 3-79647, and the absolute type.
The former comprises a fixed member, a movable member provided movably with respect to the fixed member, and an electrostatic capacity sensor for sensing the phase of a period signal that is originated according to the movement of the movable member. In this constitution, if the movable member is moved, a phase signal periodically changing with respect to the movement amount of the movable member (spindle) is sensed by a displacement sensor, as shown in FIG. 10. By counting the variation amount of this phase signal, the movement amount of the movable member is calculated from the relationship between the movement amount of the movable member and the phase period.
The latter involves sensing a plurality of phase signals having different periods that are originated according to the movement amount of the movable member. In this constitution, if the movable member is moved, two or more phase signals having different periods are sensed, so that the absolute position of the spindle is measured from the phases of those phase signals.
For example, a phase signal of long period (non-dense phase signal) and a phase signal of short period (dense phase signal) are sensed. Then, the positions at which the non-dense phase signal and the dense phase signal are situated in one period are obtained via the electric circuits (phase conversion circuit, interpolation circuit, etc.). From the relationship between these phases, at what periodicity the phase of dense phase signal takes place in the periods of the non-dense phase signal is calculated. From this result, the upper digits are calculated from the phase of the non-dense phase signal, and the lower digits are calculated from the phase of the dense phase signal. The calculated upper digits and lower digits are weighted and synthesized.
Herein, the method for specifying the periodicity that the phase of dense phase signal takes place in the periods of the non-dense phase signal involves dividing the phase of non-dense phase signal by the number of periods (steps) of the dense phase signal contained in one period of the non-dense phase signal.
Another method involves calculating the lower digits from a counting pulse obtained by counting a phase difference between the dense phase signal and the reference signal originated separately and synthesizing those lower digits with the upper digits obtained from the non-dense signal.
However, the increment type had the following problems.
(1) Since it is necessary to count a phase signal originated by the movement of the movable member, the device must be always in a counting state, while the movable member is being moved. Also, if the movable member is moved fast, the phase signal is changed at high rate, whereby the counting response rate must be increased to count this phase signal changing at high rate.
(2) Once a counting error occurs and if the user does not become aware of this error, a measurement error occurs. For correct measurement, the reference position of the movable member must be reset (zero set).
(3) Once the power is turned off, the reference position of the movable member must be set again when in use at the next time.
Also, the absolute type had the following problems.
(4) Though it is necessary to detect a phase difference between the non-dense or dense phase signals precisely, it is difficult to assure the precision of phase detection over a wide range. To calculate the absolute position, it is required to have a process for calculating where the dense phase signal is located in the periods of the non-dense phase signal by the logical operation for the phase signals having different periods, weighting each period, and synthesizing the phase signals, with the very complex operation steps.
(5) Besides, the method for counting the counting pulse with respect to the reference signal needs a synchronous modulation control, which is very intricate. To increase the measurement precision, it is possible to utilize the signals of three different modes of non-dense, intermediate and dense. However, the data processing is so complex that the display of data does not often follow the fast movement of the movable member. If the data processing is tried to make at high speed, the data processing unit is obliged to increase in size, resulting in a problem that this method is unsuitable for the measuring instrument of hand tool type.
The present invention has been achieved to solve the above-mentioned problems associated with the related art. It is an object of the present invention to provide an absolute position measuring device that can detect the absolute position correctly with a smaller size of the device and a simple constitution.
In order to accomplish the object above, the following means are adopted. According to a first aspect of the present invention, there is provided an absolute position measuring device comprises:
a main body;
a movable member provided movably in the main body;
a phase signal originating section for originating two or more phase signals of different periods in accordance with an amount of movement of the movable member; and
an arithmetical operation section for making an arithmetical operation on the phase signals to obtain an absolute position of the movable member, the arithmetical operation section including a phase signal processing section for comparing the phase signals to obtain a phase difference between the phase signals, and an absolute position calculating section for calculating the absolute position of the movable member from the phase difference obtained by the phase signal processing section,
wherein the two or more phase signals have different phase differences at different positions of the movable member in a movable area of the movable member.
With this constitution, if the movable member is moved, a phase signal having a different period is originated from the phase signal originating section, and the arithmetical operation section makes the arithmetical operation on this phase signal. At this time, the phase signal originated by the phase signal originating section is firstly processed by the phase signal processing section in the arithmetical operation section to obtain a phase difference. Since this phase difference is varied at a different position of the movable member, the absolute position of the movable member can be uniquely obtained from the phase difference. Thus, the absolute position of the movable member is calculated on the basis of the phase difference obtained by the phase signal processing section by the absolute position calculating section. This absolute position is displayed in digital format by the display section, whereby the absolute position of the movable member can be known.
In related art, the upper digits were obtained by calculating the periodicity that the dense signal is located in the periods of the non-dense signal through the arithmetical operation on the non-dense and dense phase signals. Furthermore, the lower digits were obtained from the dense signal, and the upper and lower digits were synthesized. However, with this invention, the absolute value can be uniquely obtained from the phase difference. Hence, by making the arithmetical operation section a simple constitution, the miniaturization is allowed, the fast arithmetical operation is enabled, and the cost is reduced.
According to a second aspect of the present invention, in the absolute position measuring device as defined in the first aspect of the present invention, it is preferable that the absolute position calculating section calculates a periodicity to which any one of the phase signals corresponds from a reference point on the basis of the phase difference, calculates the amount of movement of the movable member corresponding to a phase variation amount based on the periodicity, and the amount of movement of the movable member corresponding to a phase variation amount of any one of the phase signals, and calculates a total amount of movement of the movable member by synthesizing the amount of movement of the movable member based on the periodicity and the amount of movement of the movable member corresponding to the phase variation amount of any one of the phase signals, thereby calculating the absolute position of the movable member.
With this constitution, first of all, the periodicity to which any one of the phase signals corresponds from the reference point is calculated. The phase difference obtained by the phase signal processing section is varied at different position of the movable member, and thereby has a certain width for different periodicity. Hence, the periodicity of any one of the phase signals is uniquely decided as, for example, xe2x80x9cN-th periodxe2x80x9d, from the obtained phase difference. If the periodicity N is decided, the phase variation amount by which any one of the phase signals has passed up to this periodicity N is obtained. Then, the movement amount by which the movable member has moved up to the periodicity N is obtained from the periodicity N and the movement pitch per period of the movable member. Also, the movement amount by which the movable member has moved from the periodicity N to any one of the phase signals from any one of the phase signals and the movement pitch per period of the movable member. The total movement amount of the movable member from the reference point is obtained by synthesizing the movement amount by which the movable member has moved up to the periodicity N and the movement amount by which the movable member has moved from the periodicity N to any one of the phase signals.
With this invention, the phase signal directly reflected to the absolute position of the movable member is any one of the phase signals, which is required to be precise. The remaining phase signals are only employed to obtain the periodicity from the phase differences by comparison with any one phase signal, whereby an error is allowable to the extent that the periodicity is not misread. Hence, with the constitution of originating the phase signal and detecting the phase signal not requiring any precision operation, the processing steps are cut down and the cost is reduced.
According to a third aspect of the present invention, in the absolute position measuring device as defined in the first aspect of the present invention, it is preferable that the absolute position calculating section calculates a periodicity to which any one of the phase signals corresponds from a reference point on the basis of the phase difference, calculates a total phase variation amount from the phase variation amount based on the periodicity and a phase variation amount of any one of the phase signals, and calculates the absolute position of the movable member on the basis of the total phase variation amount.
With this constitution, the periodicity to which any one of the phase signals corresponds from the reference point is calculated. The phase difference obtained by the phase signal processing section is varied at different position of the movable member, and there by has a certain width for different periodicity. Hence, the periodicity of anyone of the phase signals is uniquely decided as, for example, xe2x80x9cN-th periodxe2x80x9d, from the obtained phase difference. If the periodicity N is decided, the phase variation amount by which any one of the phase signals has passed up to this periodicity N is obtained. Moreover, the total phase by which any one of the phase signals has passed from the reference point is obtained from this phase variation amount and any one of the phase signals. The absolute position of the movable member is decided from the total phase and the pitch of the movable member corresponding to one period of any one of the phase signals.
In related art, the absolute position of the movable member was obtained by synthesizing the positional data of the movable member obtained from all the non-dense and dense signals. Therefore, it was required that all the non-dense and dense phase signals were precise, and the phase signal processing was performed at high precision, resulting in the difficulty of working the parts and the higher cost.
However, with this invention, the phase signal directly reflected to the absolute position of the movable member is any one of the phase signals, which is required to be precise. The remaining phase signals are only employed to obtain the periodicity from the phase differences by comparison with any one phase signal, whereby an error is allowable to the extent that the periodicity is not misread. Hence, with the constitution of originating the phase signal and detecting the phase signal not requiring any precision operation, the processing steps are curtailed and the cost is reduced.
According to a fourth aspect of the present invention, in the absolute position measuring device as defined in the first aspect the present invention, it is preferable that the movable member is a spindle provided to be movable forth or back by rotation in an axial direction thereof, and the phase signal originating section comprises two sets of rotary encoders having a stator fixed to the main body and a rotor opposed to the stator and provided rotatably in conjunction with the rotation of the spindle, wherein the two rotors have different rotational periods, and a different phase difference at a different position of the spindle in the movable area of the spindle.
With this constitution, if the spindle is rotated, the spindle is moved forth or back in the axial direction, and each of the two rotors is rotated along with the rotation of the spindle. When the rotation of the spindle is stopped, the relative rotation phases of two rotors to the stator are detected by the stator. At this time, since the two rotors have a different phase difference at a different position of the spindle, the absolute position of the spindle is decided from this phase difference.
In this invention, two rotors are rotated along with the rotation of the spindle to acquire the phase signal, whereby there is the advantage that two rotors has a different phase difference at different position of the spindle. That is, when one rotor is changed by R periods in the movable area of the spindle, the other rotor has a period change number of R+1 or Rxe2x88x921.
According to a fifth aspect of the present invention, in the absolute position measuring device as defined in the fourth aspect of the present invention, it is preferable that the spindle is inserted independently rotatably through a center of rotation of the rotor and has two key grooves provided around the outer periphery of the spindle, the two rotors are provided with keys engaging the key grooves, and the two key grooves are provided such that the two rotors have different rotational periods, and a different phase difference at a different position of the spindle in the movable area of the spindle.
With this constitution, if the spindle is rotated, each of the two rotors is rotated by the keys engaging the key grooves of the spindle. At this time, two rotors are rotated at different rotation periods and have a different phase difference at different position of the spindle in the movable area of the spindle, whereby the absolute position of the spindle is decided from the phase difference of the rotors.
According to a sixth aspect of the present invention, in the absolute position measuring device as defined in the fifth aspect of the present invention, it is preferable that the two key grooves have different lead angles in the axial direction of the spindle.
With this constitution, since the two key grooves have different lead angles, if the spindle is rotated, the two rotors are rotated at different rotation periods. Hence, the two rotors have a different phase difference at different position of the spindle, whereby the absolute position of the spindle is decided from the phase difference of the rotors.
According to a seventh aspect of the present invention, in the absolute position measuring device as defined in fifth or sixth aspect of the present invention, it is preferable that one of the two key grooves is linearly provided in parallel to the axial direction of the spindle, and the other key groove is provided spirally around the axis of the spindle.
With this constitution, the rotor rotated by the key groove linearly provided and the rotor rotated by the key groove spirally provided can have different rotation periods.
Herein, the key groove linearly provided on the spindle is easy to work at high precision, whereas the spiral key groove is more or less difficult to work at high precision. In related art, because it was required to acquire a plurality of phase signals precisely, it was difficult that the grooves difficult to work were provided to rotate the rotors. However, in the absolute position measuring device of this invention, it is only necessary that any one of the phase signals corresponds to the rotation of the spindle correctly, and other phase signals may be as precise as deriving the periodicity of any one rotor from the phase difference. Hence, the measurement precision can be enhanced by the simple process of working the linear key grooves on the spindle precisely.
According to an eight aspect of the present invention, in the absolute position measuring device as defined in the fourth aspect of the present invention, it is preferable that the two sets of rotary encoders comprise one common stator and two rotors sandwiching the stator.
With this constitution, one phase signal is acquired from one rotor opposed to one face of the common stator, and the other phase signal is acquired from the other rotor opposed to the other face of the common stator. Hence, two different phase signals can be obtained from one common stator and two rotors to calculate a phase difference. As a result, the absolute position of the spindle is decided.
With this invention, only one stator is necessary, whereby the number of parts is reduced, the miniaturization is made, and the cost is reduced.
According to a ninth aspect of the present invention, in the absolute position measuring device as defined in the fourth aspect of the present invention, it is preferable that the two sets of rotary encoders comprise two stators and two rotors disposed between the two stators.
With this constitution, two rotors sandwiched between two stators are disposed adjacently. Then, the range where the key grooves are cut on the spindle can be narrowed. The feed screw is also cut on the spindle, but it is difficult to cut this feed screw and the key grooves in the same range. In order to miniaturize the measuring instrument itself, it is desired that the other portion than the feed screw is as short as possible. With this invention, it is possible to minimize the range where the key grooves are cut, leading to the miniaturization of the measuring instrument itself.
According to a tenth aspect of the present invention, in the absolute position measuring device as defined in the fourth aspect of the present invention, it is preferable that the two rotors consist of a first rotor and a second rotor, the spindle being inserted independently rotatably through the center of rotation of the first and second rotors, a first key is provided around an outer periphery of the spindle, the first rotor has a first key groove engaging the first key and a second key around an outer periphery thereof, the second rotor has a second key groove engaging the second key, wherein the two key grooves are provided such that the two rotors have different rotational periods, and a different phase difference at a different position of the spindle in the movable area of the spindle.
With this constitution, if the spindle is rotated, the first rotor is rotated by engagement between the first key and the first key groove. If the first rotor is rotated, the second rotor is rotated by engagement between the second key and the second key groove. Then, two rotors have a different phase at different position of the spindle in the movable area of the spindle, whereby the absolute position of the spindle is decided from the phase difference between these two rotors.
According to a eleventh aspect of the present invention, in the absolute position measuring device as defined in the tenth aspect of the present invention, it is preferable that the first key groove and the second key groove have different lead angles in the axial direction of the spindle.
With this constitution, if the spindle is rotated, two rotors are rotated at different rotational periods. That is, the rotational phase of the first rotor with respect to the spindle is defined by the lead angle of the first key groove, and the rotational phase of the second rotor with respect to the first rotor is defined by the second key groove. Hence, since the two key grooves have different lead angles, the two rotors have a different phase difference at different position of the spindle, whereby the absolute position of the spindle is decided from this phase difference.
According to a twelfth aspect of the present invention, the absolute position measuring device as defined in the tenth or eleventh aspect of the present invention, it is preferable that the first key groove is linearly provided in parallel to the axial direction of the spindle, and the second key groove is provided spirally around the axis of the spindle.
With this constitution, the first rotor rotated by the key groove linearly provided and the second rotor rotated by the key groove spirally provided have different rotational periods. That is, the first rotor is rotated at the same period as the spindle, whereas the second rotor is rotated at a different period from the first rotor.
Herein, since the first key groove is linearly provided and the second key groove is spirally provided, the same effects as in the sixth aspect of the present invention can be achieved. That is, the measurement precision can be enhanced by the simple process of working the linear key grooves on the spindle precisely.
According to a thirteenth aspect of the present invention, in the absolute position measuring device as defined in the fourth aspect of the present invention, it is preferable that the phase signal originating section comprises:
transmitting electrodes, which are provided at regular intervals along a circumferential direction of the stator, for accepting a phase signal from a phase modulator that originates a reference signal of fixed frequency shifted by every 360/n (n is a natural number);
a coupling electrode, which is provided in the rotor, for electrostatically coupling with the transmitting electrodes at a corresponding circumferential position;
a receiving electrode, which is provided in the stator, for electrostatically coupling with the coupling electrode; and
a phase signal processing section for converting an electric signal from the receiving electrode into a rotational phase of the rotor.
With this constitution, the phase modulator originates a reference signal of fixed frequency shifted by every 360/n, this phase modulated signal being sent to the transmitting electrodes of the stator. Since the transmitting electrode is electrostatically coupled at the corresponding circumferential position with the coupling electrode of the rotor, the potential appearing on the coupling electrode due to a rotational phase of the rotor is phase modulated with the reference signal. At this time, the potential appearing on the coupling electrode depends on the coupling with the transmitting electrode. The coupling electrode is also electrostatically coupled with the receiving electrode on the stator, so that a change in the potential at the coupling electrode is received by the receiving electrode. The change in the potential received by the receiving electrode is sent to the phase signal processing section for the signal processing, while a phase shift from the reference signal is detected and the relative rotation amount of the rotor to the stator is detected as the relative phase. Hence, the phase difference is obtained by comparing the relative phases, whereby the absolute position of the spindle is obtained.